Melanopsin, ganglion-cell photoreceptors, and mammalian photoentrainment.

An understanding of the retinal mechanisms in mammalian photoentrainment will greatly facilitate optimization of the wavelength, intensity, and duration of phototherapeutic treatments designed to phase shift endogenous biological rhythms. A small population of widely dispersed retinal ganglion cells projecting to the suprachiasmatic nucleus in the hypothalamus is the source of the critical photic input. Recent evidence has shown that many of these ganglion cells are directly photosensitive and serve as photoreceptors. Melanopsin, a presumptive photopigment, is an essential component in the phototransduction cascade within these intrinsically photosensitive ganglion cells and plays an important role in the retinal photoentrainment pathway. This review summarizes recent findings related to melanopsin and melanopsin ganglion cells and lists other retinal proteins that might serve as photopigments in the mammalian photoentrainment input pathway.